High-speed diodes and Schottky barrier diodes are known as semiconductor devices having a rectifying function and used for a switching power supply and the like. The switching power supply having a high conversion efficiency is adapted to rectify alternating input voltage into direct voltage, convert the direct voltage into alternating voltage having a high frequency via an ON-OFF circuit, transform the alternating voltage by means of a transformer, and then rectify the resulting voltage again to convert it into direct voltage. In this switching power supply, Schottky barrier diodes having excellent high-speed switching characteristics are used as a device for performing high-frequency rectification. Significant factors for determining the efficiency of the switching power supply include a loss in the ON-OFF circuit and a forward voltage drop that occurs in the diode that performs the high-frequency rectification. In particular, the forward voltage drop in the diode is desired to be minimized so as to achieve a sufficiently high efficiency of the power supply.
However, since Schottky barrier diodes are limited to a relatively narrow range of rated voltage as compared with pn junction diodes, a circuit including the Schottky barrier diodes needs to be designed so that a reverse voltage greater than the rated voltage is not applied to the diodes. In addition, the occurrence of surge voltage must be taken into consideration. Thus, a device whose withstand voltage is one order higher than that required in the circuit design is generally used as Schottky barrier diode. Otherwise, if a voltage, such as surge voltage, that is higher than the rated voltage is applied to the Schottky barrier diode, avalanche breakdown occurs in the Schottky barrier diode, possibly causing destroy by electric power even at small current levels. Thus, the device having a high withstand voltage is selected as the Schottky barrier diode so as to minimize a possibility of the occurrence of avalanche breakdown in the Schottky barrier diode. In the meantime, the forward voltage drop is increased with an increase in the withstand voltage, which impedes an improvement in the efficiency of the power supply.